Optimization Technical Introduction
Optimization Technical Introduction
Optimization Technical Introduction
Optimization: Introduction
Contents
Global indicators
Typical Radio Network Problem
Coverage Problem
Interference problem
Unbalanced power budget problem
Congestion problem
Optimization: Introduction
Global Indicators
4 stages of a call establishment, 2 stages
(1&2) for location update :
Optimization: Introduction
Mobile Originated Call (MOC)
(RACH)
CHANNEL REQUIRED
Indicators
IMMEDIATE ASSIGN COMMEND Establishment
IMMEDIATE ASSIGN
(AGCH)
Phase
SABM (L3 info)
AUTHENTICATION REQUEST
AUTHENTICATION RESPONSE
SETUP
CALL PROCEEDING
ASSIGNMENT REQUEST
PHYSICAL CONTEXT REQUEST
ASSIGNMENT COMMEND
(SDCCH)
SABM
ESTABLISH INDICATION
TCH UA (L3 info)
ASSIGNMENT COMPLETE
ALERTING
CONNECT
CONVERSATION
Optimization: Introduction
Global Indicators
Handover : Intracell HO
MS BTS BSC MSC
Measurement Report
MEASUREMENT RESULT
UA (L3 info)
(SDCCH)
HANDOVER PERFORMED
Optimization: Introduction
Global Indicators
Handover : Internal HO
Serving Target
MS BSC MSC
Cell Cell
Meas. Report
MEASUREMENT RESULT
CHANNEL ACTIVATION
HANDOVER ACCESS
HANDOVER DETECTION
PHYSICAL INFORMATION
UA (L3 info)
ESTABLISH INDICATION
Optimization: Introduction
Global Indicators
Handover : External HO
MS Serving Cell BSC MSC BSC Target Cell MS
MEAS_REPORT
MEAS_RESULT
HO_REQUIRED
CR (HO_REQUEST)
CC CHANNEL_ACTIVATION
CHANNEL_ACT_ACK
HO_REQUEST_ACK
HO_COMMAND (HO_COMMAND)
HO_ACCESS
HO_ACCESS
HO_DETECTION
HO_DETECTION
PHYSICAL_INFO
HO_COMPLETE
HO_COMPLETE
CLEAR_COMMAND
Cause: HO_SUCCESSFUL
Release of TCH
Optimization: Introduction
Global Indicators
Important indicators :
SDCCH Congestion
SDCCH Assignment Failure
SDCCH Drop
TCH Assignment Failure
Call Drop
Call Setup Success
Call Success
Outgoing Handover
Incoming Handover
Handover Causes Distribution
(Better/Level/Quality/MCHO/...)
Optimization: Introduction
Global Indicators
SDCCH Congestion :
MS BTS BSC
CHANNEL REQUEST
CHANNEL REQUIRED
(RACH)
(AGCH)
Optimization: Introduction
Global Indicators
SDCCH RF Failure : main causes
MS BTS BSC MSC
CHANNEL REQUEST
CHANNEL REQUIRED
(RACH)
(AGCH)
Optimization: Introduction
Global Indicators
SDCCH RF Failure : main causes : Phantom RACH
1. Noise decoding
GSM 05.05: “0.02% of RACH Frame can be
decoded without error without real input signal ”
(this extra-load has no impact for the system)
BCCH not combined: 156 phantom RACCH/hour
(or about 0.08 Erlang SDCCH)
BCCH combined: 83 phantom RACH/hour
During period with low real traffic (night), high rate
of phantom RACH
No impact for subscriber
But impact on indicator SDCCH RF access failure
Optimization: Introduction
Global Indicators
SDCCH RF Failure : main causes : Phantom RACH
Optimization: Introduction
Global Indicators
SDCCH RF Failure : main causes : Phantom RACH
Optimization: Introduction
Global Indicators
SDCCH Drop :
MS BTS BSC MSC
Optimization: Introduction
Global Indicators
TCH assign failure : Congestion
MS BTS BSC MSC
ASSIGNMNET REQUEST
Optimization: Introduction
Global Indicators
TCH assign failure : Radio
ASSIGNMENT REQUEST
PHYSICAL CONTEXT REQUEST
ASSIGNMENT COMMEND
(SDCCH)
TCH
Radio Problem
ASSIGNMENT FAILURE
Radio interface failure
Optimization: Introduction
Global Indicators
TCH assign failure : BSS
ASSIGNMENT REQUEST
PHYSICAL CONTEXT REQUEST
ASSIGNMENT COMMEND
(SDCCH)
BSS Problem
Optimization: Introduction
Global Indicators
TCH Drop : Radio
Optimization: Introduction
Global Indicators
TCH Drop : BSS
MS BTS BSC MSC
Optimization: Introduction
Global Indicators
TCH Drop : BSS
CLEAR REQUEST
BSS problem O&M intervention
Radio interface failure
Optimization: Introduction
Global Indicators
TCH Drop : Handover
Optimization: Introduction
Global Indicators
Intracell HO Failures
Congestion
Reversion to Old Channel (ROC)
Drop Radio
BSS problem
Optimization: Introduction
Global Indicators
Intracell HO Failures : Congestion
Serving
MS BSC MSC
BTS
Measurement Report
MEASUREMENT RESULT
No Free TCH/SDCCH
Optimization: Introduction
Global Indicators
Intracell HO Failures : Reversion to Old Channel
Serving Serving
MS MSC
BTS BSC
ASSIGNMENT CMD ASSIGNMENT COMMEND (old channel)
UA (new channel)
X
SABM (new channel)
UA (new channel)
X
SABM (old channel)
ESTABLISH INDICATION (old channel)
UA (old channel)
Optimization: Introduction
Global Indicators
Internal HO Failures : Reversion to Old Channel
Serving Target
MS BSC MSC
Cell Cell
HO CMD HANDOVER COMMAND
HANDOVER ACCESS
HANDOVER DETECTION
PHYSICAL INFORMATION
UA (L3 info)
ESTABLISH INDICATION
HANDOVER COMPLETE
X
SABM
ESTABLISH INDICATION
UM
HO FAILURE HO FAILURE
Optimization: Introduction
Global Indicators
Internal HO Failures : Drop
Serving Target
MS BSC MSC
Cell Cell
Meas. Report
MEASUREMENT RESULT
CHANNEL ACTIVATION
Clear_request
Clear_command
Optimization: Introduction
Global Indicators
External HO Failures : Congestion
HO_FAILURE
Cause: no radio resource available
HO_REQUIRED_REJECT
HO_FAILURE
Cause: terrestrial circuit already allocated
HO_REQUIRED_REJECT
Requested trerestrial resource unavailable
BSS not equipped
Optimization: Introduction
Global Indicators
External HO Failures : Reversion to Old Channel
CC CHANNEL_ACTIVATION
CHANNEL_ACT_ACK
HO_REQUEST_ACK
HO_COMMAND (HO_COMMAND)
HO_ACCESS
X
HO_ACCESS
X
SABM
ESTABLISH_INDICATION
UA
CLEAR_COMMAND
Radio interface fail:
Reversion to old channel Release connection
Optimization: Introduction
Global Indicators
External HO Failures : Drop
CC CHANNEL_ACTIVATION
CHANNEL_ACT_ACK
HO_REQUEST_ACK
HO_COMMAND (HO_COMMAND)
HO_ACCESS
X
HO_ACCESS
X
SABM
X
SABM
X
SABM
X
CLEAR_COMMAND
Radio interface fail Release connection
Optimization: Introduction
Global Indicators
Important indicators : SDCCH Congestion Rate
Optimization: Introduction
Global Indicators
Important indicators : SDCCH Assign Failure Rate
Optimization: Introduction
Global Indicators
Important indicators : SDCCH Drop Rate
Optimization: Introduction
Global Indicators
Important indicators : TCH Assign Failure/Congestion Rate
Definition:
TCH Assign Failure Rate: Rate of RTCH seizure
failed (BSS problem, radio access problem) during
normal assignment procedure over the total amount
or RTCH request for normal assignment procedure
TCH Assign Congestion Rate: Rate of RTCH not
allocated during normal assignment due to
congestion on Air interface
Optimization: Introduction
Global Indicators
Important indicators : Call Drop Rate
Optimization: Introduction
Global Indicators
Important indicators : Call Success Rate (BSS view)
Optimization: Introduction
Global Indicators
Important indicators : Incoming Handover Success Rate
Optimization: Introduction
Global Indicators
Important indicators : Handover Cause Distribution
Optimization: Introduction
Typical Radio Problems
i. Coverage Problem
ii. Interference problem
iii.Unbalanced power budget
problem
iv. Congestion problem
Optimization: Introduction
Typical Radio Problems
QoS indicators
Coverage predictions
Customer complaints
Drive tests
Abis/A traces
Other teams information (NSS statistics)
Optimization: Introduction
i. Coverage Problem
Bad coverage :
A network facing coverage problems has bad
RxLev. RxQual can be bad at the same time.
Sometimes the RxLev can look OK on the
street (i.e. from drivetest) but coverage inside
the buildings can be poor due to building
losses. Building losses can range from 10 to
30 dB or more.
Optimization: Introduction
i. Coverage Problem
Indications :
Customers complain about dropped calls and/or
“No Network”
OMC QoS indicators
– TCH failure rate
– Call drop rate
– Low proportion of better cell HO
– High rate of DL & UL Level HO (possibly also
DL/UL quality HO)
A interface indicators
– High rate of Clear Request message, cause radio
interface failure
Drive Test
– Poor RxLev ( < -95 dBm)
Optimization: Introduction
i. Coverage Problem
Investigation and Optimization :
If actual coverage is not the one predicted
by planning tool
– Check antenna system (azimuth, crossed
sector?)
– Check prediction in the tool (EIRP, Clutter
Type, Obstructing building?). Most of the time
the prediction will be incorrect, as it is only a
computer simulation
– Increase or decrease antenna down-tilt
– Check BTS output power set in the OMC-R
(BS_TXPWR_MAX: should always be set at
0)
Optimization: Introduction
i. Coverage Problem
Investigation and Optimization :
If actual coverage OK compared to
predicted ones
– Improve coverage by adding Macro site,
Microcell site (Border or Shadow areas), or
IBC site (In-building coverage).
– Use parameter setting ex. Bi-layer Handover.
Optimization: Introduction
i. Coverage Problem
Example on thresholds :
Optimization: Introduction
ii. Interference Problem
Interference :
Optimization: Introduction
ii. Interference Problem
Indications :
Customers complain about bad speech quality
(noisy calls) and/or call drops
OMC QoS indicators
– SDCCH/TCH Drop
– Low proportion of better cell HO
– High rate of DL/UL quality HO and interference
HO
– Low HO success rate
A interface indicators
– High rate of Clear Request message, cause radio
interface failure
Drive Test
– RxQual > 4 with RxLev > -85 dBm
Optimization: Introduction
ii. Interference Problem
Typical causes :
GSM interferences
Co-channel
adjacent
Optimization: Introduction
ii. Interference Problem
Level
Co-channel interference :
C/I = Carrier-to-interference ratio
+12 dB is taken into account for +12 dB
Optimization: Introduction
ii. Interference Problem
Co-channel interference :
Indications
Downlink and/or Uplink Interference
High rate of quality handover, call drop, and call
setup failure
Investigation
Drive test analysis
Lumpini: Co-channel checking
Check prediction to verify which cell could be
causing the interference
Frequency plan C/I < 12 dB
Optimization: Introduction
ii. Interference Problem
Co-channel interference :
Optimization
Antenna optimization
Change frequency
Reduce BTS power (not an option in TA Orange
network)
Concentric cell
Optimization: Introduction
ii. Interference Problem
Co-channel interference: Optimization: Concentric cell
Concept Conclusion from Alcatel trial on
Orange’s network (Suburban area)
Optimization: Introduction
ii. Interference Problem
Adjacent channel interference :
C/A = Carrier-to-adjacent ratio
-6 dB is taken into account for Alcatel ( -9 dB
according to GSM standard).
Level
f(cell1) f(cell2)
-6 dB
E.g.
f(cell1)= Ch.573
f(cell2)= Ch.574
f(cell2)=f(cell1)+1 Frequency
Optimization: Introduction
ii. Interference Problem
Adjacent channel interference :
Indications
Downlink and/or Uplink Interference
High rate of quality handover, call drop, and call
failure
Investigation
Adjacent HO statistics (if they are defined as
neighbors). If they often handover, then adjacent
channel interference could be a problem.
Drive test analysis
Lumpini: Adjacent channel checking
Cell coverage prediction
Frequency plan C/I < -6 dB
Optimization: Introduction
ii. Interference Problem
Optimization: Introduction
ii. Interference Problem
Interference : Forced Directed Retry
MS should connect to Cell_A, but no TCH
available.
MS connects to Cell_B with forced directed
retry
MS is emitting at high level (due to long
distance from Cell_B) Cell_B: Ch.573
Optimization: Introduction
ii. Interference Problem
Non GSM interference :
Other mobile network
Inter-modulation with GSM BTS/MS receiver
Other RF interferers
Radar
Shop anti-theft mechanism
Medical devices
Other man-made radio transmission
Illegal usage of GSM frequencies (e.g. illegal
imported devices, illegal usage of other
organization, ...)
...
Optimization: Introduction
iii. Unbalance power budget Problem
A cell facing unbalanced power budget problems
presents a too high path-loss difference between UL
and DL (often DL>UL)
Lower Rx diversity gain of cross polarized antenna in
rural area compared to dense area
Crossed feeders
Traffic not directly in boresight of antenna, where
cross-polar discrimination is lower
Loose connectors
Faulty antenna, feeder, TMA
Rule : try to have delta as small as possible to avoid
access network possible only in 1 direction (usually
BTS->MS : OK and MS->BTS : NOK), i.e. uplink
limited
Optimization: Introduction
iii. Unbalance power budget Problem
Indicators :
OMC QoS indicators
– High ratio of Uplink level Handover cause
– Low incoming HO success rate
– DL level looks OK for HO, but UL is too weak when
HO is attempted
– Degradation of TCH failures and OC call drop
indicators
A interface indicators
– High rate of Clear Request message, cause radio
interface failure
O&M Alarms
– Voltage Standing Wave Ratio (VSWR)
– TMA
Optimization: Introduction
iii. Unbalance power budget Problem
Investigation :
Abis monitoring:
– ABS(delta Path loss) > 10
» Check if problem is occurring for 1 TRX or
all.
– ABS(delta path loss) = ABS(UL Path loss – DL
Path loss)
– UL Path loss = MS transmitted power – measured
received UL level
– DL Path loss = BTS transmitted power – combiner
loss – measured received DL level
– Large difference between uplink and downlink
level triggered HO.
Use PM observation counter (in OMC-R)
– RT11
Optimization: Introduction
iii. Unbalance power budget Problem
Optimization: Introduction
iv. TCH congestion Problem
TCH Congestion :
Optimization: Introduction
iv. TCH congestion Problem
Indications :
Customers complain about “Network busy”
OMC QoS indicators
– High “TCH Congestion rate”, “RTCH
Assignment fail-Congestion”
– Low “incoming Intra/Inter BSC HO success
rate” (no TCH available)
– High “Directed Retry rate or Congestion Relief
(Motorola)” if activated
A interface indicators: “BSS Congestion failure in
OC”
– High rate of Assignment Failure message, No
radio resource available
Drive Test
– Handover to better cell is slow, or never occurs
Optimization: Introduction
iv. TCH congestion Problem
Optimization :
Special events :
– Foreseeable: football match, important
meeting, Exhibition, etc.
» Add some extra TRXs
» Add special mobile BTSs (Cell on
Wheel)
» Activate HR (may not always be
possible due to BSC capacity
limitations)
– Not foreseeable: car crash on the highway
Optimization: Introduction
iv. TCH congestion Problem
Optimization :
Optimization: Introduction
iv. TCH congestion Problem
Optimization :
Optimization: Introduction
Problems and responsible parties
Coverage problem :
If the measured RxLev does not match
the prediction
– Check the prediction, check panoramic
pictures for obstructions
– Field Operation team to check physical
configuration (tilt, azimuth, antenna
height...). Also check for faults in the
antenna system (VSWR, sweep antennas,
DTF test, etc.)
– If Field Operation find no problem,
antenna optimization may be required (if
necessary)
Optimization: Introduction
Problems and responsible parties
Coverage problem :
Optimization: Introduction
Problems and responsible parties
Interference problem :
Optimization team to identify the interference
source and request Spectrum Management team
to correct it in case of internal source (new
frequency planning...)
Optimization: Introduction
Problems and responsible parties
Optimization: Introduction